Ionomeric membranes are used for a number of modern applications including battery technologies, fuel cell technologies, reverse osmosis, filtration, and the like.
In battery applications, such membranes are used as separators. Battery separators are porous sheets that are interposed between an anode and cathode in a fluid electrolyte. For example, in lithium ion batteries, lithium ions (Li+) move from the anode to the cathode during discharge. The battery separator acts to prevent physical contact between the electrodes while allowing ions to be transported. Typical prior art separators include microporous membranes and mats made from nonwoven cloth. Battery separators are ideally inert to the electrochemical reactions that occur in batteries. Therefore, various polymers have been used to form battery separators.
In the case of fuel cells, membranes are used as ion conductors. For example, in proton exchange membrane type fuel cells, hydrogen is supplied to the anode as fuel and oxygen is supplied to the cathode as the oxidant. The oxygen can either be in pure form (O2) or air (a mixture of O2 and N2). PEM fuel cells typically have a membrane electrode assembly (“MEA”) in which a solid polymer membrane has an anode catalyst on one face, and a cathode catalyst on the opposite face. The anode and cathode layers of a typical PEM fuel cell are formed of porous conductive materials, such as woven graphite, graphitized sheets, or carbon paper to enable the fuel to disperse over the surface of the membrane facing the fuel supply electrode. Each electrode has finely divided catalyst particles (for example, platinum particles), supported on carbon particles, to promote oxidation of hydrogen at the anode and reduction of oxygen at the cathode. Protons flow from the anode through the ionically conductive polymer membrane to the cathode where they combine with oxygen to form water, which is discharged from the cell. The MEA is sandwiched between a pair of porous gas diffusion layers (“GDL”), which in turn are sandwiched between a pair of non-porous, electrically conductive elements or plates. The plates function as current collectors for the anode and the cathode, and contain appropriate channels and openings formed therein for distributing the fuel cell's gaseous reactants over the surface of respective anode and cathode catalysts. In order to produce electricity efficiently, the polymer electrolyte membrane of a PEM fuel cell must be thin, chemically stable, proton transmissive, non-electrically conductive and gas impermeable. In typical applications, fuel cells are provided in arrays of many individual fuel cells in stacks in order to provide high levels of electrical power. In order to perform well, polyelectrolyte membranes require hydration provided by the water vapor transfer, humidification membranes described herein.
Reverse osmosis membranes are used for many applications including water purification and the concentration of ethanol from aqueous alcohol mixtures. Membranes of this type are made from reconstituted cellulose which suffers from low temperature applications, biodegradation, and chemical instability.
Accordingly, the present invention provides improved methods of making membranes with graded pore sizes that are useful in filtration, humidification, battery and fuel cell applications.